The field of the invention is magnetic resonance imaging (MRI) methods and systems. More particularly, the invention relates to a method and system for synchronizing the initiation of an MRI data acquisition to the arrival of a signal-enhancing contrast agent.
Magnetic resonance imaging is commonly employed for a variety of imaging applications including medical imaging. When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field Bz), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. A net magnetic moment Mz is produced in the direction of the polarizing field, but the randomly oriented magnetic components in the perpendicular, or transverse, plane (x-y plane) cancel one another. If, however, the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or xe2x80x9ctippedxe2x80x9d, into the x-y plane to produce a net transverse magnetic moment M1, which is rotating, or spinning, in the x-y plane at the Larmor frequency. The degree to which the net magnetic moment M1 is tipped, and hence the magnitude of the net transverse magnetic moment M1 depends primarily on the length of time and the magnitude of the applied excitation field B1. A signal is emitted by the excited spins, and after the excitation signal B1 is terminated, this signal may be received and processed to form an image.
One particular type of MRI measurements which are known as xe2x80x9cpulsed MRI measurementsxe2x80x9d are divided into a period of excitation and a period of signal emission. Such measurements are performed in a cyclic manner in which the MRI measurement is repeated many times to accumulate different data during each cycle or to make the same measurement at different locations in the subject.
When utilizing MRI to produce images, a technique is employed to obtain MRI signals from specific locations in the subject. Typically, the region which is to be imaged is scanned by a sequence of MRI measurement cycles which vary according to the particular localization method being used. The resulting set of received MRI signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques. To perform such a scan, it is, of course, necessary to elicit MRI signals from specific locations in the subject. This is accomplished by employing magnetic fields (Gx, Gy, and Gz) which have the same direction as the polarizing field B0, but which have a gradient along the respective x, y and z axes. By controlling the strength of these gradients during each MRI cycle, the spatial distribution of spin excitation can be controlled and the location of the resulting MRI signals can be identified.
MRI data for constructing images can be collected using one of many available techniques, such as multiple angle projection reconstruction and Fourier transform (FT). Typically, such techniques comprise a pulse sequence made up of a plurality of sequentially implemented views. Each view may include one or more MRI experiments, each of which comprises at least an RF excitation pulse and a magnetic field gradient pulse to encode spatial information into the resulting MRI signal, as well as a data acquisition window.
The preferred embodiments of the invention will be described in detail with reference to a variant of the well known FT technique, which is frequently referred to as xe2x80x9cspin-warpxe2x80x9d. The spin-warp technique is discussed in an article entitled xe2x80x9cSpin Warp NMR Imaging and Applications to Human Whole-Body Imagingxe2x80x9d by W. A. Edelstein et al., Physics in Medicine and Biology. Vol. 25, pp. 751-756 (1980).
The spin-warp technique employs a variable amplitude phase encoding magnetic field gradient pulse prior to the acquisition of MRI spin-echo signals to phase encode spatial information in the direction of this gradient. In a two-dimensional implementation (2DFT), for example, spatial information is encoded in one direction by applying a phase encoding gradient (Gy) along that direction, and then a spin-echo signal is acquired in the presence of a read-out magnetic field gradient (Gx) in a direction orthogonal to the phase encoding direction. The read-out gradient present during the spin-echo acquisition encodes spatial information in the orthogonal direction. In a typical 2DFT pulse sequence, the magnitude of the phase encoding gradient pulse Gy is incremented (xcex94Gy) in the sequence of views that are acquired during the scan to produce a set of MRI data from which an entire image can be reconstructed.
The use of three-dimensional versions of the spin-warp method (3DFT) is finding wider use in clinical applications. In the 3DFT implementation, spatial information is encoded in two directions by applying phase encoding gradients along both directions and acquiring the MRI signal in the presence of a readout gradient along the third direction. According to one type of 3DFT scan, one of the phase encoding gradients (e.g. Gz) is stepped through all its values, and for each Gz step, the other phase encoding gradient (e.g. Gz) is stepped through all its values. It has been found, however, that this scan technique can be problematic when it is important to initiate the scan sequence at a particular instant, for example, when a contrast agent is used.
During magnetic resonance imaging examinations, such as magnetic resonance angiography examinations, it is often useful to employ an exogenous, intravenously contrast agent to enhance details of the vasculature of interest. In particular, the arterial segments are usually of most interest. The contrast agent is typically used to shorten the longitudinal relaxation time for T1 weighted images, thus greatly enhancing the signal intensity of the vasculature through which the contrast agent has perfused. In such studies, the highest quality image data is acquired when the contrast agent initially passes through the vasculature of interest and the image data deteriorates as the contrast agent begins to exit the structure of interest. For example, when imaging arteries, the image data deteriorates when the contrast agent begins to exit the arteries and enter the veins, because undesirable contrast enhancement in the veins obscures the arteries. Since 3DFT scans require considerable time to acquire all the phase encoding views needed for an image reconstruction, at least some of the MRI data is acquired under less than ideal circumstances, for example, during undesirable veinous enhancement.
To address this problem, U.S. Pat. No. 5,122,747 describes a scan technique commonly known as centric view ordering or centric encoding. Centric encoding scans (ky, kz) space in a spiral pattern starting at the center (or origin) of (ky, kz) space (ky=0, kz=0) and working outward. Since the central views contain the majority of structural information about the object, these central views can be positioned to be acquired at the optimal moment during the procedure and the peripheral views are acquired later. Equal y-axis and z-axis fields of view are assumed and hence the spacings xcex94ky and xcex94kz between samples are equal. Because of this equal spacing, the sampling path is a square spiral from which this phase encoding scheme takes its name. U.S. Pat. No. 5,912,557 describes a related scan technique commonly known as elliptic centric encoding. This technique can be used in situations when the fields of view along the two phase encoding axes are significantly different. According to this technique, each (ky, kz) space sample point is ranked according to its distance from the origin of (ky, kz) space. The closer points are ranked higher and appear earlier on the ordered list. The MRI data is then acquired in an order that is determined by this list, resulting in a sampling path that is an elliptic spiral from which this phase encoding scheme takes its name. In the simplified case where the fields of view are the same, elliptic centric encoding yields a sample path that is the same as that yielded by ordinary or square centric encoding.
Although these techniques have proven to be extremely useful, it is still desirable to accurately synchronize initiation of the centric data acquisition to the arrival of the contrast agent. If the data acquisition begins too soon, then the initial image data has a poor signal-to-noise ratio due to the contrast agent not yet having arrived at the structure of interest. If the data acquisition begins too late, then the image data has a poor signal-to-noise ratio due to a reduction in the concentration and effectiveness of the contrast agent as the contrast agent passes out of the structure of interest. Thus, if the centric data acquisition is started too early or too late relative to the arrival time of the contrast agent, the examination may not show the desired results, namely, signal enhancement of the structure of interest. Additionally, if the scan is started too late, then the contrast agent begins passing into other tissues or structures (e.g., veins), and the other tissues or structures become more prominent in the resulting image thereby corrupting the imaging of the arterial vasculature. Typically it is desirable to capture image data from only the structure of interest, because the presence of additional structure in the displayed image clutters the image and makes viewing the structure of interest more difficult.
In an exemplary embodiment of the invention, a method of synchronizing initiation of a magnetic resonance image (MRI) acquisition to the arrival of a contrast agent in a structure of interest comprises performing a first MRI scan that acquires a first three-dimensional MRI data set from the structure of interest. This step is repeated at least until the first MRI scan indicates that the contrast agent has arrived in the structure of interest. The method also comprises performing a second MRI scan that acquires a second three-dimensional MRI data set from the structure of interest, this step being initiated based on when the first MRI scan indicates that the contrast agent has arrived in the structure of interest.
In another exemplary embodiment of the invention, a method of synchronizing initiation of a magnetic resonance image acquisition to the arrival of a contrast agent in a structure of interest comprises repetitively performing a partial MRI scan. This step includes performing a first plurality of MRI pulse sequences that acquires sample MRI data for a first three-dimensional MRI data set. The method also comprises terminating the repetitively performing step and completing a full MRI scan after the partial MRI scan indicates that the contrast agent has arrived in the structure of interest. This step includes performing a second plurality of MRI pulse sequences that acquire sample MRI data for a second three-dimensional MRI data set.
Advantageously, the preferred embodiments makes it possible to accurately synchronize MRI image acquisition to the arrival of a contrast agent at the structure of interest. As a result, image quality of the structure of interest is enhanced, while the undesirable presence of unrelated structure in the acquired image is simultaneously minimized. Additionally, exam efficiency is maximized because only a single imaging procedure is required. Once the contrast agent is detected, it is possible to switch from a partial acquisition mode used to detect the contrast agent to a complete acquisition mode used for full fidelity 3D imaging. The manner of data acquisition is the same for both modes, thereby allowing both contrast agent detection and full fidelity 3D image acquisition to occur in a single imaging procedure.